Gas-assisted injection molding of large panels with sequential gating

ABSTRACT

Molding of relatively large articles such as automobile body parts by gas-assisted injection molding with sequential gating of injected thermoplastic resin into the mold cavity. Elongated ribs in the parts form gas channels and strengthening ribs for the parts. Structural parts are made by adhesively joining two such parts made by gas-assisted injection molding through a combination of hollow ribs and joining flanges at sides of the parts.

This application claims the benefit of provisional application Ser. No.60/040,339 filed Mar. 7, 1997.

This invention relates to injection molding. In one of its aspects, theinvention relates to injection molding of relatively large panels. Inanother of its aspects, the invention relates to injection molding ofrelatively large panels with a good surface finish. In another of itsaspects, the invention relates to injection-molded articles withintegral strengthening ribs and a good surface finish.

In yet another of its aspects, the invention relates to a process andapparatus for injection molding relatively large articles with integralhollow support ribs and a good surface finish.

BACKGROUND OF THE INVENTION

Relatively large articles such as automobile parts and curbside refusecollection carts have been integrally molded with injection molding androtational molding processes. Attempts are presently being made to makelarger and larger molded plastic parts in the transportation industry.Plastic parts have the advantage of light weight, corrosion resistanceand lower cost.

Thermosetting polyester filled with chopped fibers have been compressionmolded into relatively large sheets or panels. The surface finish is notparticularly good. Decorative panels are typically painted.

Resin transfer molding (RTM) has been used to make some external bodyparts. A glass or graphite preform is positioned in a mold and a liquidthermosetting resin is injected into the mold. The thermosetting resinsolidifies and forms the body of the part. These parts typically need astructural support and have a relatively poor surface finish. However,these parts have traditionally been painted because the surface finishhas not been satisfactory enough for use without painting.

The resin transfer molded parts are not recyclable in that thethermosetting resins cannot be remelted and reused. Thus, reject partsmust be scrapped and sent to the landfill. This scrapping of rejectparts increases the ultimate cost of the acceptable parts. Even forthose parts which are satisfactory, the parts must be sent to thelandfill when the vehicle is scrapped or when they are damaged.

Vehicle manufacturers are designing more and more parts with ultimatedisposal in mind. Thus, it is becoming more and more important to designautomotive bodies with materials which can be recycled. According,thermosetting materials are not particularly desirable.

Thermoplastic resins with glass fibers have been extruded in sheet form.Glass fibers have also been used as a laminate in thermoplastic resinsheet form. The sheets are then compression molded to a particularshape. These parts are recyclable in that the thermoplastics can becomminuted and recycled. Compression molding has limitations withrespect to certain shapes. For example, compression molded parts cannotbe drawn very deeply and thus must be of relatively shallowconfiguration. Further, any holes in the sheet are required to be madewith a secondary operation, thus adding cost to the finished product.Further, the parts are not particularly strong and require structuralreinforcements to be used in a vehicle body, for example. Further, thesurface finish is not particularly good.

Injection molding of thermoplastic resin has been used for many smallarticles. Larger articles require a larger clamping tonnage for the moldhalves due to the pressure with which the thermoplastic resin isinjected and forced to the limits of the mold cavities. Some largearticles have been made but the parts themselves are not particularlystructural. For example, curbside refuse carts in 96-gallon size havebeen injection molded in relatively large presses. However, these cartsdo not have close tolerance requirements. Further, fenders and doorshave been made in an injection-molding process. The fenders and doors,however, are not load bearing and have little structural integrity.These panels must be attached to the frame of the car body. Further, theouter surfaces are always painted because of surface flaws whereexternal surface finish is important. In one instance, a bumper fasciahas been made by injection molding and not painted. The bumper fasciawas not structural.

The injection molding of larger articles requires multiple drops(gates). Typically, all gates open simultaneously. The use of multiplegates typically produces multiple knit lines. When parts exceed fivefeet in any one dimension, the problem is exacerbated.

It has been proposed to make automotive bodies by molding portions ofthe frame and skin from plastic and joining the frame and skin togetherby bonding. These parts are generally dish-shaped and nest within eachother. There are difficulties in forming the bonding surfaces withoutadding significant weight or without expensive mold designs.

Another problem with injection molding larger articles is that the sizeof the articles is limited by clamping tonnage. The larger the projectedarea of the article, the greater the clamping tonnage. Machines whichhave very large clamping tonnage are very expensive and difficult tohouse. Extremely large clamping tonnage injection molding machines areextremely rare because of high cost.

Still another problem with injection molding of large articles isdistortion due to uneven densities of the thermoplastic materialthroughout the articles. When higher molding pressures are used,thermoplastic resin near a gate will tend to pack more densely than theresin near the ends of the mold cavity. As a result, a largeinjection-molded article will sometimes warp due to uneven density ofpacking of the thermoplastic material. In parts in which fit and finishare important, i.e., low dimensional tolerances, warping and packing isunacceptable. When material has high fiber content or filler,orientation can be a serious problem.

Klobucar et al. in U.S. Pat. No. 5,162,092 disclose a process forinjection molding a thermoplastic backing or other synthetic resin to acarpet layer by suspending the carpet layer in a mold cavity, injectingthermoplastic resin into the mold cavity and injecting an inert fluid,such as nitrogen, into the mold at a relatively low pressure to assistin distributing the thermoplastic resin throughout all points in themold. The thermoplastic resin as well as the inert fluid is injectedinto a rib cavity in the mold to form an internal runner or rib having ahollow portion substantially along the width of the article which ismolded.

Relatively large dash mats have been injection molded from a filledthermoplastic resin in a mold involving multiple gates wherein the gateshave been sequentially operated to sequentially distribute thethermoplastic in the mold. These dash mats are not structural and have apoor surface finish. They are not typically visible to the automobileoccupant.

SUMMARY OF THE INVENTION

According to the invention, a relatively large article is made with anintegral structural support and a quality surface finish in a processwhich comprises the steps of:

injecting molten thermoplastic resin into a mold cavity at a firstlocation and flowing the thermoplastic resin from the first location toa second location spaced from the first location;

injecting molten thermoplastic resin into a mold cavity at a secondlocation substantially simultaneously with the arrival of the moltenthermoplastic from the first location at the second location;

discontinuing the flow of molten thermoplastic resin to the firstlocation; and

injecting an inert gas under pressure into the mold cavity to assist indistributing the molten thermoplastic resin to the edges of the moldcavity.

The thus molded article is cooled at least to a solid state andpreferably to a dimensionally stable state, the inert gas is vented fromthe mold cavity, the mold is opened and the molded article is removedfrom the mold cavity.

Preferably, the flow of molten thermoplastic resin to the first locationis discontinued at about the time the injection of molten thermoplasticresin into the mold cavity at the second location is commenced. Further,the commencement of the injection of the inert gas under pressure intothe mold cavity generally takes place subsequent to the discontinuationof the injection of molten thermoplastic resin.

In a preferred embodiment of the invention, the mold cavity has anelongated rib cavity and the molten thermoplastic resin and inert gasare injected into the rib cavity. The rib cavity thus forms a hollowstructural rib in the article to rigidify the article. Further, the ribcan form relatively wide bonding surfaces on the article for bondinginjection-molded articles together.

Further according to the invention, the molten thermoplastic resin flowsfrom at least the second location to a third location spaced from thefirst and second locations and molten thermoplastic resin is injectedinto the mold at the third location substantially simultaneously withthe arrival of the molten thermoplastic resin at the third location. Theflow of molten thermoplastic resin to the second location isdiscontinued, preferably at about the time the injection of the moltenthermoplastic resin into the mold cavity at the third location iscommenced. In a preferred embodiment, the inert gas is further injectedunder pressure into the mold cavity to assist in the distribution ofmolten thermoplastic resin to the edges of the mold cavity. Thecommencement of the injection of the inert gas typically takes placegenerally about the time the flow of molten thermoplastic resin into themold cavity is discontinued. Sets of multiple first, second, third andother resin injection gates can be sequentially opened according to theinvention to inject thermoplastic resin into the mold cavity. Typically,each set of injection gates are opened as thermoplastic resin reaches aset of injection gates from another set of injection gates through themold cavity.

Further according to the invention, an article is made in accordancewith the processes according to the invention described above. Thearticle has a quality decorative surface finish without the necessity ofpainting or otherwise covering the surface. Further, the article,although relatively large, for example, in excess of four feet,preferably in excess of five feet, is relatively free from distortiondue to relatively even distribution of the thermoplastic resin. Due tothe sequential gating of the molten thermoplastic resin in the mold andthe use of a gas assist to distribute the molten thermoplastic resinwithin the mold cavity, uneven densities of material due to packing issubstantially avoided. Further, uneven fiber orientation is minimized infiber filled compositions due to the multiple drops of the moltenthermoplastic material during the molding operation. Preferably, pairsof articles are bonded together to form a structural body, for example,an automobile body portion. The structural body has rigid hollowpassageways formed in part by bonding together portions of injectionmolded articles which together define the hollow passageways.Preferably, the rigid passageways form a rigid spine in the structuralbody whereby the structural body has a smooth finished skin, yet has arelatively high strength-to-weight ratio due to the structural spine.The rigid channels preferably have at least one hollow gas channel whichforms a bonding surface and provides structural rigidity to the rigidhollow passageways. The hollow gas channels are formed by injecting gasinto a mold cavity during injection molding of the molded articles.Preferably, there are two hollow gas channels in each rigid hollowpassageway and the hollow gas channels are disposed diametricallyopposed from each other across the hollow passageways.

Further according to the invention, an apparatus for molding arelatively large article comprises a pair of mold halves defining a moldcavity therebetween and at least first and second injection conduits,spaced from each other, in one of the mold halves for injecting moltenthermoplastic resin into the mold cavity. Injection valves or gates areprovided in the first and second injection conduits to control the flowof molten thermoplastic resin into the mold cavity. A controller isprogrammed to control the first and second injection valves to initiallyopen the first valve and close the second valve during an initial timeperiod in the injection cycle. The controller is programmed to open thesecond valve about the time when the molten thermoplastic resin arrivesat the second injection conduit through the mold cavity from the firstinjection conduit. Further, a gas-injection conduit with a gas controlvalve is provided in one of the mold halves for injecting an inert gasinto the mold cavity, preferably in a rib cavity. The controller isprogrammed to control the gas control valve to control the injection ofinert gas into the mold cavity to force the molten thermoplastic resinto the edges of the mold cavity, preferably about the time the flow ofmolten thermoplastic resin to the mold cavity is discontinued. Thecontroller is further programmed to discontinue the flow of moltenthermoplastic material into the mold cavity through the first injectionconduit at about the time the injection of molten thermoplastic materialinto the mold cavity through the second injection conduit is commenced.

The mold cavity preferably comprises an elongated rib cavity. The firstand second injection conduits and the gas-injection conduit terminate inthe rib cavity. The injection of gas into the rib cavity hollows out therib and packs the thermoplastic resin at the sides of the rib cavity toenhance the strength and rigidity of a molded rib in the resultingarticle.

In a preferred embodiment of the invention, the one mold half has athird injection conduit spaced from the first and second injectionconduits and has a third injection valve or gate to control the flow ofthermoplastic resin therethrough. The controller is programmed to openthe third injection valve or gate substantially simultaneously with thearrival of the molten thermoplastic resin from the first or secondinjection conduit at the third injection conduit and is programmed toclose the second injection valve to discontinue the flow of moltenthermoplastic resin to the second injection conduit.

In one embodiment of the invention, at least one of the first and secondmold halves has pressure sensors to detect the pressure of thermoplasticresin at several locations in the mold cavity. The pressure sensors areoperably connected to the controller to provide inputs to the controlleras to the arrival of the molten thermoplastic resin at least at thesecond and third injection conduits.

DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the accompanyingdrawings in which:

FIG. 1 is a perspective view of a large molding according to theinvention;

FIG. 2 is a partial sectional view taken along lines 2--2 of FIG. 1;

FIG. 3 is a partial sectional view taken along lines 3--3 of FIG. 2;

FIG. 4 is a sectional view of a mold for making the molding illustratedin FIGS. 1-3;

FIG. 5 is a schematic diagrammatic representation of a control systemfor use in molding articles according to the invention;

FIG. 6 is a flow chart illustrating a process according to theinvention;

FIG. 7 is a perspective view of an automotive body part manufactured bya process according to the invention;

FIG. 8 is a partial view of the body part illustrated in FIG. 7 and anadditional cover body part also manufactured according to a processaccording to the invention;

FIG. 9 is a plan view of an article made in accordance with theinvention;

FIG. 10 is a sectional view taken along lines 10--10 of FIG. 9;

FIG. 11 is a sectional view taken along lines 11--11 of FIG. 9; and

FIG. 12 is a sectional view taken along lines 12--12 of FIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings and to FIGS. 1-3 in particular, there is showna large molded article 12 having a central rib 14 and branch ribs 16. Anopening 17 is formed in the article 12. As illustrated in FIG. 2, therib 14 has a hollow channel 18. As illustrated in FIG. 3, the branchribs 16 have a hollow channel 20.

The article 12 is relatively large and has a finished Class A surface 22opposite the surface containing the ribs 14 and 16. The surface 22 ismolded with finish quality and can be used in unpainted condition forlarge parts, for example, automotive parts such as fenders, doors andother external body parts. The article 12 is relatively thin in crosssection but the ribs 14 and 16 provide structural support for thearticle 12. Thus, the article 12 is self-supporting due in part to thestructural nature of the ribs 14 and 16 and can be mounted throughstructural flanges (not shown) to the supporting frame of a vehicle, forexample.

The article 12 is made in a mold illustrated in FIG. 4 to whichreference is now made. A mold has mold halves 24 and 26 which form amold cavity with cavity surfaces 24 and 50, respectively. A rib cavitysurface 30 which, for example, forms the central rib 14 or a branch rib16 is provided in the cavity 28. Typically, multiple rib cavities areformed in the mold surface 24. First, second and third drop (injection)conduits 32, 34 and 36, respectively, are formed between the exterior ofthe mold half 24 and the rib cavity surface 30 and terminate in thefirst, second and third spacer locations in the rib cavity. First,second and third air-injection conduits 38, 40 and 42 are formed betweenthe exterior surface of the mold half 24 and the rib cavity 30. First,second and third pressure sensors 40, 46 and 48, respectively, aremounted in the mold half 24 at the surface of the rib cavity 30 and areconnected by wires (not shown) to a controller. Temperature sensors (notshown) can also be provided in the mold adjacent the cavity 28 tomeasure the temperature of the mold cavity.

A manifold 52 is mounted on top of the mold half 24 and has an interiordistribution channel 54 coupled at one end to an injection port 56 andto first, second and third delivery ports 58, 60 and 62, respectively. Afirst gate valve 64 is mounted in the first delivery port 58. A secondgate valve 66 is mounted in the second delivery port 60. A third gatevalve 68 is mounted in the third delivery port 62.

First, second and third gas supply conduits 70, 72 and 74 are connectedrespectively to the first, second and third air-injection conduits 38,40 and 42 for delivery of pressurized gas to the rib cavity 30. Anextruder 76 is coupled to the manifold 52 to deliver on demand fluidthermoplastic resin at an elevated temperature. The extruder 76 can beany conventional thermoplastic extruder capable of heating thermoplasticpellets and extruding molten thermoplastic resin at elevatedtemperatures and pressures. Examples of thermoplastic resins includehigh-density polyethylene, polypropylene, thermoplastic polyester,polycarbonates, acrylics, blends and alloys thereof. Typically, thetemperature of the thermoplastic resin will be dependent on theprocessing temperature of the materials. The pressure of thethermoplastic resin during extrusion will be dependent on the materialsand the nature of the molded articles.

Referring now to FIG. 5 which is a schematic representation of thecontrol system of the invention, where like numerals have been used todesignate like parts. Injection conduits 32, 34 and 36 each have a gatevalve 64, 66 and 68, respectively, which are connected to a controller88 through control lines 100. Each of the pressure sensors 44, 46 and 48are connected to the controller 88 through control lines 98 and providesinputs 96 to the controller 98. The controller also has other inputs 90,92 and 94 at which other set points are installed into the controller.The first, second and third gas supply conduits 70, 72 and 74 areconnected to an inert gas supply source 84 which is supplied with aninert gas source under pressure, for example, from a compressor or froman inert gas tank. The first, second and third gas supply conduits 70,72 and 74 have, respectively, a first gas control valve 78, a second gascontrol valve 80 and a third gas control valve 82. These gas controlvalves are connected to the controller through control lines 102.

The controller is programmed to open the first gate valve 64 when thesecond gate valve 66 and the third gate valve 68 are closed to injectmolten thermoplastic resin into the mold cavity through the first dropor injection conduit 32. The controller 88 is further programmed to openthe second gate valve 66 at a time substantially simultaneous with thearrival of molten thermoplastic resin through the mold cavity from thefirst drop conduit 32 and to thereafter close the first gate valve 64.The controller is preferably programmed to close the first gate valve 64substantially simultaneously with the opening of the second gate valve66. The arrival of the molten thermoplastic resin through the moldcavity at the second drop or injection conduit 34 can be sensed by thesecond pressure sensor 46.

The controller 88 is further programmed to open the third gate valve 68substantially simultaneously with the arrival of molten thermoplasticresin through the cavity from the first or second drop conduits 32 and34. The presence of the molten thermoplastic resin at the third dropconduit 36 can be detected by pressure sensor 48 and applied as an inputthrough one of the input ports 96 to the controller 88. The controller88 is further programmed to close the second gate valve 66.

The controller 88 is further programmed to close the gate valve 68 aftera predetermined time interval and to open the gas control valves 78, 80and 82 to inject gas under pressure through the third gas-injectionconduit 42 into the mold cavity to assist in distributing the moltenthermoplastic material to the corners of the mold.

The controller 88 is further programmed to close the gas control valves78, 80 and 82 after the molded article has cooled at least to a solidstate and preferably to a stable state. The controller further can beprogrammed to open the mold after a predetermined cooling periodwhereupon the molded article can be removed from the mold cavity.

The operation of the mold illustrated in FIG. 4 to make the article 12illustrated in FIGS. 1-3 will now be described with references to FIGS.4, 5 and 6. The mold halves 24 and 26 are closed and clamped togetherthrough a substantial clamping force with conventional clamps (notshown). Molten thermoplastic resin under high pressure is extruded froman extruder 76 into the distribution channel 54. The first gate valve 64is opened by the controller 88 and molten thermoplastic resin is forcedthrough the first delivery port 58 and through the first drop orinjection conduit 32 into one location of the mold cavity. As the moltenthermoplastic resin enters the mold cavity, it will be begin flowing ina radial direction from the first drop conduit 32. Typically, the moltenmaterial will fill the comers and pockets around the first drop conduitand flow towards the second drop conduit 34. As the molten thermoplasticresin flows in the mold cavity, it is cooled at the mold cavity surfaceand begins to freeze along the interface of the mold cavity and themolten thermoplastic resin. The center of the molten thermoplasticresin, however, remains molten and is pushed away from the first dropconduit 32.

The pressure in the rib cavity 30 is detected by the first, second andthird pressure sensors 44, 46 and 48 which relay control signals to thecontroller 88 through control lines 98 and inputs 96. When the moltenthermoplastic resin reaches the second drop conduit 34, the pressuredetected by the second pressure sensor 46 will increase, therebyindicating the presence of the molten material at the second dropconduit 34. At this time, the controller closes the first gate valve 64and opens the second gate valve 66 so that molten thermoplastic resinflows through the second drop conduit 34 to a second location in themold cavity.

The molten thermoplastic resin which flows through the second dropconduit 34 will flow to the extent possible radially outwardly from thesecond drop conduit 34 in the cavity 28 and 50 and along the rib cavity.The presence of the molten thermoplastic material at the third dropconduit 36 will be detected by the third pressure sensor 48. A controlsignal will be transmitted to the controller 88 from the third pressuresensor 48 through one of the control lines 98. When the moltenthermoplastic material reaches the third drop conduit 36, the secondgate valve 66 is closed by the controller 88 and a third gate valve 68is opened by the controller 88.

After the molten thermoplastic resin has been injected into the moldcavity through the third drop conduit 36 for a predetermined period oftime or, alternatively, until the pressure detected by the thirdpressure sensor 48 reaches a predetermined value, the controller willopen the gas control valves 78, 80 and 82 and pressurized gas issupplied through the gas supply conduits 70, 72 and 74 to the gasinjection conduits 38, 40 and 42 and into the mold cavity at the ribcavity surface 30. Pressurized gas entering the mold cavity through thegas-injection conduits 38, 40 and 42 forces the molten thermoplasticresin along the rib cavity to form a hollow channel therein and forcesthe thermoplastic resin to the ends of the mold cavity. At or before thetime that the gas is injected into the mold cavity through thegas-injection conduits 38, 40 and 42, the controller 88 closes the thirdgate valve 68. The pressure in the first, second and third gas-injectionconduits 38, 40 and 42, respectively, is maintained until such time asthe molded article is adequately cooled. The controller then closes thefirst, second and third gas control valves 78, 80 and 82. The gaspressure in the mold cavity is then vented and the mold cavity isopened. The molded article can then be removed from the mold cavity. Thegas pressure in the first, second and third gas-injection conduits 38,40 and 42 is maintained in the mold cavity until such time as thethermoplastic resin has solidified and the part is substantially cooledbeyond the point at which shrinkage will occur.

The molding process takes place by sequential introduction ofthermoplastic resin. The sequencing of the gate operation can bemonitored with pressure detectors 44, 46 and 48 or can be controlledthrough a timing program which opens the gates in a predeterminedsequence which approximates the arrival of the molten thermoplasticresin at a downstream injection port. The sequencing of the gates forthe molten thermoplastic resin provides a continuous flow of resinthroughout the mold without interfacing of two or more wave fronts ofmolten thermoplastic resin. Thus, the surface formed in the mold cavityis smooth and free of knit lines.

Further, because the material flows sequentially from one gate to thenext, excessive pressure and "packing" of the thermoplastic resin in thearticle is minimized. The molded articles are relatively free fromwarpage as well as from surface blemishes. The use of gas injectionprovides a vehicle for molding larger articles with a given tonnagebecause of the distribution of the thermoplastic resin to the ends ofthe mold cavity with the pressurized gas rather than solely throughincreased molding pressure of the extruder.

The invention has been described with respect to FIGS. 1-6 in arelatively simple manner to illustrate the invention. Although onlythree sequentially operated injection gates or drop conduits have beenshown and described for purposes of simplicity, typically, sets ofmultiple gates are operated in sequence using the same principlesdiscussed above to make large and complex articles. Two or more sets ofgates can be used to make the larger articles. There may be as many asten sets of multiple gates which are operated sequentially to form thelarge article.

Referring now to FIG. 7, there is shown a relatively large automotivestructural body frame 120 of a dish-shaped configuration integrallymolded in one piece by a process and apparatus according to theinvention. The structural body frame 120 has a side portion 122, a topportion 124, a bottom portion 126, a front portion 128 and a backportion 130. These portions are integrally molded together in one pieceand form one-half of a structural body frame.

The side portion 122 comprises a fender frame 132 having an inner sidepanel 134 defining a wheel well opening and a wheel well panel 136. Afront frame member 138 extends upwardly from a front portion of theframe 120 and joins an A pillar frame member 142. A connecting panel 140extends between the front frame member 138 and the wheel well panel 136.A rib 144 is formed in the front frame member 138. The rib 144 is hollowand has a cross-sectional configuration, for example, as illustrated inFIGS. 2 and 3. A hollow rib 146 of similar nature is formed in the Apillar frame member 142. A rib 148 of similar configuration to the rib144 is formed in the wheel well panel 136. Apertures 150 are formed ineach of the frame portions at strategic locations.

A side roof frame element 152 extends rearwardly from the upper portionof the A pillar frame member 142 and to a C pillar 164 at the rearportion of the body frame 120. A hollow rib 153 extends along the sideroof frame element 152 and has a configuration similar to the ribsillustrated at FIGS. 2 and 3. A B pillar frame element 154 extendsdownwardly from the side roof frame element 152 to a rear fender frame162. A hollow rib 156 of the type illustrated in FIGS. 2 and 3 is formedin the B pillar 154. A side bottom frame 160 having a rear portion ofthe hollow rib 148 extends between the front wheel well panel 136 andthe rear fender frame 162. C or rear pillar 164 extends between the sideroof frame element 152 and a side rear frame element 166. Hollow ribs ofthe type illustrated in FIGS. 2 and 3 are also formed in the C or rearpillar 164 and in the side rear frame element 166. A door opening 158 isdefined by the front wheel well panel 136, the A pillar frame element142, the side roof element 152, the B pillar 154, a front portion of therear wheel frame 162 and the side bottom frame element 160. An opening168 is formed by a rear portion of the side roof frame element 152, theC or rear pillar 164, an upper portion of the side rear frame element166 and an upper portion of the rear wheel frame 162.

The top portion 124 of the frame comprises U-shaped roof frame members170, 172 and 174, all of which extend laterally from the side roof frameelement 152. Each of these roof frame members 170, 172, and 174 have ahollow rib of the same nature illustrated in FIGS. 2 and 3.

The bottom portion 126 of the body frame 120 has a spare tire wheelwell(?) 178, a floor portion 180, a rear fender well 176 and a frontwheel well 182. The front portion 128 of the body frame 120 is formed bya U-shaped frame member 184 having a front deck panel 186 and U-shapedframe member 188 and 190. The U-shaped frame members 184, 188 and 190extend laterally from the front frame member 138.

The back portion 130 of the body frame 120 has a rear panel 192 and arear window opening 194.

The body frame 120 forms a half of a structural frame for a vehicle.This very large part is molded by a process and apparatus according tothe invention. A mirror image frame (not shown) part can also be formedaccording to the invention and joined to the body frame 120 illustratedin FIG. 7 to form a complete vehicle body frame.

Referring now to FIG. 8 where like numerals have been used for likeparts, there is shown a structural cover panel 200 formed in a dishshape by a U-shaped side frame 202 having U-shaped B pillar element 204,U-shaped A pillar element 206 and U-shaped C pillar element 208.U-shaped roof frame elements 210, 212 and 214 extend laterally from theU-shaped side roof frame 202. The dish-shaped body frame 120 nestswithin the cover panel 200.

The structural cover panel is made in accordance with a process andapparatus according to the invention. The external surface of the coverpanel 200 has a very fine finish and may not need to be painted orotherwise finished. Further, each of the U-shaped frame elements whichcomprise the cover panel 200 have an internal hollow rib of the samenature as, for example, ribs 144 and 146 illustrated in FIG. 7. Further,the interior of the cover panel 120 is provided with a series of sockets(not shown) which project downwardly from an interior surface of thepanel toward the structural body frame 120. These sockets are inregistry with the apertures 150 and receive fasteners (not shown) whichextend through the apertures 150 from beneath the body frame 120 and aresecured in the sockets in the cover panel 200 to rigidly secure thecover panel 200 to the structural body frame 120. One or more structuralcover panels are mounted to the body frame 120 in similar fashion toprovide a structural finished external surface for the body frame. Eachof the cover panel 200 and the body frame are structural in nature. Theyare rigidified by the ribs 144, 146, 148, 153, etc. which are hollow toreduce the weight without appreciably reducing the rigidity. The coverpanel 200 and the body frame 120 can be bonded together with adhesivesor chemically bonded together as well as or in lieu of mechanicalfasteners. The combination of the cover panel 200 and the body frame 120forms a rigid structural frame which has a clear exterior finish whichneed not be painted or otherwise finished. The ribs form runners for theplastic and yet form a part of the final structure. The two body parts,when secured together, have significant bending and torsional rigiditywithout any metal parts. Although the runners add some weight to thepart, the part is still light in weight because of the hollow channelswithin the runners.

Preferably, the two body parts have walls which, when joined together,form structural rigid passageways which preferably are interconnected toform a rigid spine for the assembled body parts. The rigid channels areformed at least in part by the hollow ribs which further form bondingsurfaces for bonding the parts together in a manner described below withreference to FIGS. 9-12. The resulting body parts have a highstrength-to-weight ratio, yet have a superior exterior surface finish.

The parts have significant structural integrity and have a fit andfinish which enables the parts to be mated together for bonding withoutdistortion. The low tolerances to which the parts are made is achievedby sequential gating and the gas-assisted injection molding of the ribs.

Referring now to FIG. 9, there is shown a plan view of an article 230made in accordance with a process according to the invention. Thearticle 230 has an upper shell 232 and a lower shell 234. The uppershell 232 has an outer rim 236 and a curved inner rim 238 coupled to astraight inner rim 240. The upper shell 232 further has a curved innerrim 242 which is coupled with a straight inner rim 244 to define aninterior opening. An interior opening is also formed between the curvedinner rim 238 and the straight inner rim 240. Fastener openings 246 and248 are formed in the upper shell for fasteners.

Referring now to FIG. 10, the upper shell 232 has a depending tube 252aligned with the fastener openings 248 and cylindrical opening 254therethrough. The outer rim 236 is formed as a rib having a lowerbonding surface 256, an inner surface 258 and a hollow gas channel 259.In like manner, the curved inner rim 238 is formed as a rib having aninner surface 260, a lower bonding surface 262 and a hollow gas channel264.

The lower shell 234 has an outer L-shaped flange 266 forming an innerL-shaped bonding surface 268 and a web 272. An outer flange 274 has anupper bonding surface 276 connected to the web 272 through an L-shapedweb 278. A bolt opening 280 is formed between the webs 272 and 278. Atubular extension 282 having an inner cylindrical surface 284 is alignedwith the bolt opening 280.

The upper shell 232 and the lower shell 234 are typically bondedtogether through a suitable adhesive at the bonding surfaces 262 and 268on one side and the bonding surfaces 256 and 276 at the other side. Abolt (not shown) can extend through the bolt openings 248 and 280 toalso secure the upper and lower shells 232 and 234 together. Asillustrated in FIG. 10, a rigid article having a hollow passageway isformed by bonding together the upper shell 232 and the lower shell 234,with the walls forming the passageway being rigidified through the ribsformed by the outer rim 236 and the inner rim 238. The ribs furtherdefine bonding surfaces through which the shells can be bonded together.The weight of the rim is significantly reduced and the cost of the resinis significantly reduced by the hollow gas channels 259 and 264. The gaschannels are formed by gas assist in the injection-molding process inthe manner described above. As illustrated in FIG. 10, the two hollowribs are positioned diametrically opposite one another across the hollowpassageway defined by the walls of the upper and lower shells 32 and 34.

FIG. 11 illustrates a variation in the shell structures described abovewith reference to FIG. 10. Turning now to FIG. 11, the straight innerrim 244 is formed as a rib defined by an inner surface 288, a lowerbonding surface 290 and a hollow gas channel 292. On the other hand, thestraight inner rim 240 is of relatively uniform thickness incross-sectional area and forms a bonding surface 286.

The lower shell 234 includes a central web 294 connected to a rib 296defined by an outer surface 298, an upper bonding surface 300 and aninner surface 302. A hollow gas channel 304 is formed in the rib 296.The central web 294 is connected to an outer flange 308 through aconnecting web 306. The outer flange 308 forms an upper bonding surface310 and is relatively uniform in cross-sectional thickness.

The upper shell 232 and the lower shell 234 are secured together throughadhesives at the bonding surfaces 286 and 300 at one side and thebonding surfaces 290 and 310 at the other side of the cross-sectionalshape illustrated in FIG. 11. Like the hollow shell assembly illustratedin FIG. 10, the configuration illustrated in FIG. 11 is a rigid hollowstructure which has a high strength-to-weight ratio. The wide bondingsurfaces are formed by the ribs 296 and the rib at the straight innerrim 244. The gas channels 292 and 304 reduce the weight and reduce theamount of material needed to form the shapes. The resulting structure isvery rigid in nature, yet is light in weight, having a highstrength-to-weight ratio.

Referring now to FIG. 12, there is shown yet another configuration whichis similar to the configuration shown in FIG. 10 except that the ribsare formed at the bottom shell 234 rather than on the top shell. Asillustrated in FIG. 12, the upper shell 232 has an outer flange 312 ofrelatively uniform cross-sectional thickness and forms a lower bondingsurface 314. The curved inner rim 238 is formed in relatively uniformcross section, terminating in an outer flange 316 which forms a lowerbonding surface 318.

The lower shell 234 has a central web 320 which is connected to an outerrib 322 defined by an outer surface 324, an upper bonding surface 326and an inner surface 328. A hollow gas channel 330 is formed in the rib322.

An inner rib 332 is formed by an outer surface 334, an upper bondingsurface 336 and an inner surface 338. A hollow gas channel 340 is formedin the inner rib 332. A bolt opening 342 having a tubular extension 344with an inner cylindrical surface 346 extends inwardly in alignment withthe bolt hole 246. A tubular extension 350 having an inner cylindricalsurface 348 extends from the bolt hole 256. A bolt (not shown) can bepositioned within the bolt hole 246 and 342 to secure the portion of theupper shell 232 to the portion of the lower shell 234 illustrated inFIG. 12. Preferably, the upper shell 232 is bonded to the lower shell234 in part through adhesives which are positioned on the bondingsurfaces 314 and 326 on one side and to bonding surfaces 336 and 318 onthe other side of the structure illustrated in FIG. 12. The ribs 322 and330 form wide bonding surfaces for the adhesive, adding strength andrigidity to the bonded assembly. The gas channels 330 and 340 reduce theamount of material and the weight of the ribs without a loss of strengthor rigidity. The hollow gas channels 330 and 340 as well as the hollowchannels 304 and 292 are formed during the injection-molding processwith a gas assist in a manner which has been described above. Further,the use of the hollow gas channels 330, 340, etc. conveniently formsbonding surfaces while synergistically increasing the strength of theresulting structural frame. The gas channels 330, 340, etc., eliminatethe need for expensive molds for forming bonding surfaces on both sidesof the shell.

The inner and the outer shells made according to the invention arejoined together to provide the structural integrity needed for a rigidframe. The inner and outer shells can be joined by mechanical fastenerssuch as screws and rivets, or by welding, such as heat welding andsolvent welding, or by an adhesive bonding system. Adhesive bonding ispreferred over other joining techniques. Adhesive bonding provides acontinuous bond line and thus avoids stress concentration as one wouldhave in mechanical joints. Also, adhesive bonding has gap fillingcapability between the inner and the outer shell which might otherwisebe difficult to obtain in welding. Adhesive selection will depend on thethermoplastic resin substrate. Common adhesives includes epoxies,acrylates, and polyurethanes. The adhesives are usually thermosetting innature and need to be cured to obtain strength. Although adhesives cancure at ambient temperatures, heat can be applied to the bond line toaccelerate the curing rate. Heat can be applied in one of several wayssuch as hot air, infrared, microwave, induction, etc.

It is envisioned that the thermoplastic resins used to make the innerand the outer shell can be different materials. To obtain the higherimpact requirement and better surface finish, the outer shell materialcan have less or no reinforcing material. It is preferred that, in thesecircumstances, the thermoplastic resin of the inner and outer shellmaterial remain same or compatible such that joining of the shells canbe accomplished with one adhesive.

The bonded assemblies illustrated in FIGS. 10-12 are made inexpensivelythrough a gas-assist injection-molding process, with the resultingstructure having a high strength-to-weight ratio, yet having a superiorsurface finish and being formed relatively inexpensively. Mechanicalfasteners can be used where desired to join the two injection-moldedparts together to form a rigid, integral article. The configuration ofthe hollow shell structure illustrates the type of hollow channelsformed in assembled body parts according to the invention as, forexample, illustrated in FIGS. 7 and 8.

Although the invention has been described with respect to sequentialinjection of thermoplastic into a mold, sets of multiple gates aretypically opened in sequence to simultaneously distribute plastic to anumber of different gates simultaneously. However, knit lines areavoided on the surface due to a configuration of sequential gating ofthe thermoplastic material.

The parts according to the invention are relatively large, for example,greater than eight feet in one dimension.

The parts made according to the invention are relatively large but canbe made with a minimal clamping tonnage because of the use ofgas-assisted injection molding and sequential gating. Thus, machine costis minimized.

The parts made according to the invention are typically isotropic,regardless of whether the thermoplastic is filled with fibers. Thisisotropic nature of the parts is achieved by multiple drops withsequential gating and gas-assisted injection molding. Because of theprocess described above, the large parts are made with a relativelyuniform density. Uneven packing of the polymer is eliminated by thesequential gating as well as the gas-assisted injection molding process.Thus, the parts are structural in nature, have a very fine surfacefinish, have dimensional stability resulting in good fit of the partsand are essentially free of warpage. Further, the properties of themolded parts are essentially isotropic.

Nearly all types of thermoplastic resins, whether crystalline oramorphous, can be utilized to mold parts using the process described inthis invention. Examples of crystalline polymers include polyolefins,polyamides, polyesters, polyaryletherketones, polyoxymethylene polymers,liquid crystal polymers, etc., and examples of amorphous polymersinclude polycarbonates, acrylics, polystyrenes, etc. Copolymers such asstyrene-acrylonitriles and terpolymers such asacrylonitrile-styrene-acrylic and acrylonitrile-butadiene-styrene canalso be used in this invention. Also, alloys and blends of variousthermoplastics described above can be used in this invention. Examplesof common alloys and blends include Xenoy from General Electric, whichis an alloy of polycarbonate and polybutylene terephthalate, andHivalloy from Montell U.S.A. Inc., which is an alloy of polypropyleneand polystyrene. Preferred thermoplastic resins should have good flowproperties for the ease of processing the large parts and adequatestructural integrity dictated by the part application. In general, toobtain higher stiffness and higher heat distortion temperature, thesethermoplastics will be reinforced with one or more reinforcing agents.Common reinforcing agents include fibrous materials such as glassfibers, polymeric fibers and natural fibers such as jute, and mineralssuch as mica and talc. A preferred reinforcing agent is glass fiber with10-20 micron diameter and 3-4 mm in length. Longer glass fibers about 12mm in length can also be used to further improve the properties of thethermoplastic material.

Structural frame parts as illustrated in FIGS. 7 and 8 were madeaccording to a process and apparatus according to the invention using aHoechst Celanese PET polymer having 15 percent weight glass fill.

The parts were manufactured in an injection molding machine with a 9000ton clamp unit and 200 pound shot capacity (in polypropylene). The partsweighed 65 pounds and measured 100"×52"×15".

The mold used a heated manifold to convey the material from the machineto seven hydraulic actuated valve gates allowing sequencing and/orsimultaneous flow. The mold measured 96"×156" and weighed 125 tons.

The polymer is a polyethylene terephthalate polymer which was molded ata temperature in the range of 520° F. to 560° F. The mold was heated ina temperature range of 200° F. to 250° F. The gate sequence 1,3,4,6,5produced parts essentially free of distortion, with good fitcharacteristics and high-quality surface finish which did not requirepainting for decorative purposes and with cavity pressures, measured infourteen separate locations, some examples below:

top of the B-pillar (gate 2) 4,000 psi

top of the C-pillar (gate 1) 5,000 psi

inner floor pan (B-pillar) 5,000 psi

inner floor pan (C-pillar) 2,800 psi

bottom of the B-pillar (gate 5) 2,200 psi

edge of the floor pan (B-pillar) 3,500 psi

edge of the floor pan (C-pillar) 3,500 psi

The inner part employed 20 gas entry locations (nozzles), delay time andhold time of the gas to each nozzle allowing the use of all nozzlessimultaneously and/or sequentially. Cycle time was as low as 120 sec.and as high as 200 sec.

In another example, a Montell polyolefin, Hivalloy 066, was injectionmolded using a process and apparatus according to the invention to makeframe parts of the nature illustrated in FIGS. 7 and 8 using asequencing process of gates 1,2,3 on simultaneously then 4,5,6 onsimultaneously. The Hivalloy 066 material is a 35 percent glass-filledpolypropylene/polystyrene alloy. It was molded at a temperature in therange of 400° F. to 460° F., with mold temperatures in the range of 100°F. and at a pressure in the ranges shown below:

top of the B-pillar (gate 2) 3,700 psi

top of the C-pillar (gate 1) 3,700 psi

inner floor pan (B-pillar) 3,000 psi

inner floor pan (C-pillar) 2,700 psi

bottom of the B-pillar (gate 5) 3,000 psi

edge of the floor pan (B-pillar) 2,100 psi

edge of the floor pan (C-pillar) 2,000 psi

When the parts were run conventionally, using all gates in the openposition and injecting no gas (although the parts did already containthe gas channels used for flow), the parts thus molded were impossibleto fill completely, were warped, exhibited bums at the weld lines, andcaused the mold to open during the process due to excessive moldpressures in a range of 7,500 psi to 15,000 psi.

With the invention, very large structural articles can be manufacturedwith a very clear surface finish. The clear surface can be used asexternal automotive parts, for example, which need not be painted. Thesurface is clear enough to provide a good decorative surface finishwithout painting or other cosmetic work.

The parts made according to the invention can be very complex, havenumerous curves and corners, using multiple ribs and multiple drops andgas-injection ports. Each of the ribs can have one or more drops forinjection of thermoplastic materials, each of which is typicallyaccompanied by a gas-injection port. The gating sequence is determinedto control the flow of thermoplastic material away from a firstinjection port sequentially to the ends of the mold cavity withoutinterfacing of flow of thermoplastic resin from multiple drops. Partsmeasuring a length in excess of four feet, preferably in excess of fivefeet, can be made according to the invention.

The invention thus minimizes material usage, strengthens the moldedarticles through the integral ribs and provides a surface which has ahigh quality finish. Tonnage is minimized due to the sequential gatingand gas-injection process. Further, windows, doors and other openingscan be integrally molded into the article without a secondary operation.Structural supports are integrally formed and the weight of thestructural supports is minimized with the gas channel. Further, thenumber of drops needed to mold large articles of this type can bereduced with the use of sequential gating and gas injection.

Reasonable variation and modification are possible within the scope ofthe foregoing disclosure and drawings without departing from the spiritof the invention which is defined in the accompanying claims.

What is claimed:
 1. A method of molding a relatively large article in amold cavity comprising the steps of:injecting molten thermoplastic resininto the mold cavity at a first location and flowing the thermoplasticresin from the first location to a second location spaced from the firstlocation; sensing the arrival of the molten thermoplastic resin at thesecond location; injecting molten thermoplastic resin into the moldcavity at the second location substantially simultaneously with thearrival of the molten thermoplastic resin from the first location at thesecond location; controlling the step of injecting the moltenthermoplastic resin into the mold cavity at the second location inresponse to the sensed arrival of the thermoplastic resin at the secondlocation; discontinuing the flow of molten thermoplastic resin to thefirst location; subsequent to the arrival of molten thermoplastic at thesecond location, injecting an inert gas under pressure into the moltenthermoplastic resin in the mold cavity to assist in distributing themolten thermoplastic resin to the edges of the mold cavity; cooling thethermoplastic resin at least to a solid state; venting the gas from themold cavity; and opening the mold and removing the thus-molded articlefrom the mold cavity.
 2. A method of molding a relatively large articleaccording to claim 1 wherein the flow of molten thermoplastic resin tothe first location is discontinued at about the time the injection ofthe molten thermoplastic resin into the mold cavity at the secondlocation is commenced.
 3. A method of molding a relatively large articleaccording to claim 2 wherein the commencement of the injection of inertgas under pressure into the mold cavity takes place about the time theflow of molten thermoplastic resin to the first location isdiscontinued.
 4. A method of molding a relatively large articleaccording to claim 3 wherein the mold cavity has an elongated rib cavityand the inert gas is injected into the rib cavity, thereby forming ahollow rib in the article.
 5. A method for molding a relatively largearticle according to claim 4 and further comprising the steps of:flowingthe molten thermoplastic resin in the mold cavity from the first orsecond location to a third location spaced from the first and secondlocations and injecting molten thermoplastic resin into the mold cavityat the third location substantially simultaneously with the arrival ofthe molten thermoplastic resin at the third location from the first orsecond locations; and discontinuing the flow of molten thermoplasticresin to the second location.
 6. A method for molding a relatively largearticle according to claim 5 wherein the flow of molten thermoplasticresin to the second location is discontinued at about the time theinjection of the molten thermoplastic resin into the mold cavity at thethird location is commenced.
 7. A method for molding a relatively largearticle according to claim 6 wherein the step of injecting an inert gasunder pressure into molten thermoplastic resin in the mold cavitycomprises injecting the inert gas at spaced positions in the mold.
 8. Amethod of molding a relatively large article according to claim 7 andfurther comprising the step of discontinuing the flow of moltenthermoplastic resin to the third location.
 9. A method of molding arelatively large article according to claim 8 wherein the commencementof the injection of the inert gas under pressure into the mold cavitytakes place about the time the flow of molten thermoplastic resin to thethird location is discontinued.
 10. A method of molding a relativelylarge article according to claim 1 wherein the mold cavity has anelongated rib cavity and the molten thermoplastic resin and the inertgas are injected into the rib cavity, thereby forming a hollow rib inthe article.
 11. A method of molding a relatively large articleaccording to claim 1, wherein the sensing step comprises sensing thepressure in the mold at the second location.
 12. A method of molding arelatively large article according to claim 1, wherein the sensing stepcomprises sensing the heat in the mold at the second location.
 13. Amethod of molding a relatively large article according to claim 1,wherein the thermoplastic resin is PET filled with glass fibers.
 14. Amethod of molding a relatively large article according to claim 1,wherein the thermoplastic resin is a glass-filledpolypropylene/polystyrene alloy.